FIG. 1 shows a converter arm of the type described in the above-mentioned publications. The converter arm is intended to feed a load L, represented as being essentially inductive, e.g. a motor, from a DC voltage source delivered between a positive rail VR+ and a negative rail VR-. It essentially comprises:
two main switches S1 and S2, e.g. of the IGBT type, enabling an outlet S leading to the load L to be connected respectively either to the rail VR+, or to the rail VR-; PA1 two free-wheel diodes D1 and D2 enabling current through the load L to be maintained, and respectively coupling the outlet S either to positive rail VR+ or to the negative rail VR-; PA1 two snubber capacitors C1 and C2 between the outlet S and, respectively, the positive rail VR+ and the negative rail VR-; and PA1 an auxiliary circuit CA comprising, in particular, an auxiliary inductor LA in series with the two auxiliary switches T1 and T2 that conduct unidirectionally in opposite directions, e.g. thyristors or circuits that are functionally equivalent, coupled between the outlet S and the midpoint M of a capacitive voltage divider DC; the voltage divider itself comprising two divider capacitors CV1 and CV2 connected in series between the positive and negative rails VR+ and VR-, and operating at said midpoint M commonly at a voltage that is halfway between the voltages of the positive and negative rails VR+ and VR-.
In that circuit, the components S1, D1, C1, T1, and CV1 are paired respectively with the components S2, D2, C2, T2, and CV2, i.e. they show respectively the same electrical characteristics on all occasions. A control circuit (not shown) measures voltages and currents at various points of the converter comprising the above arm, and in application of an appropriate program, controls triggering of the main switches S1 and S2 and of the auxiliary switches T1 and T2 by acting on the control electrodes thereof which are sketched in the figure, e.g. at ec1, thereby causing them to operate in a manner which is described below.
To illustrate the operation of the circuit, in particular in the case of a DC--DC converter, the arm of FIG. 1 operates as a chopper, and it is assumed that it begins in a state in which none of the switches is conductive and in which a current I.sub.L is maintained through the load L by the free-wheel diode D2 which is then conducting. As a result, and ignoring the threshold voltage of the diode D2, the outlet S is at the same potential as the rail VR-, e.g. 0 volts. The capacitors CV1 and CV2 have the same capacitance, and they are charged to the voltage that exists between the rails VR+ and VR-, referred to below as +V, such that the midpoint M is at the middle voltage +V/2.
The circuit begins to switch over in a first step in which the auxiliary switch T1 is triggered by the control circuit. It becomes conductive and the auxiliary inductor LA is powered between +V/2 (ignoring the threshold of T1) and 0 V (ignoring the threshold of D2); the current I.sub.A that passes through it increases linearly. This current is subtracted from the current I.sub.L passing through the diode D2.
When I.sub.A =I.sub.L, the diode D2 ceases to conduct. The outlet S is no longer coupled to the rail VR-. The auxiliary inductance LA is then in series with the capacitors C1 and C2 which are themselves in parallel with each other as seen from the inductor. Current oscillation then begins between the inductor and the capacitors. In the first half-cycle of this oscillation, the midpoint of the snubber capacitors C1 and C2, i.e. the outlet S, goes from 0 to +V volts.
The switch S1 is then triggered, without voltage across its terminals, and it then carries the current I.sub.L. The inductor LA, now connected between the point M at the potential +V/2 and the outlet S which is maintained at the potential +V via the switch T1, now sees the current I.sub.A that is passing through it decrease linearly. When it drops to zero, the auxiliary switch T1 becomes non-conductive. That is why a thyristor type switch is used, since that type of switch has the property of switching off when the current flowing through it drops to zero, or when any equivalent combination of components is used, e.g. a transistor or an IGBT in series with a diode.
In the opposite direction, switchover for returning to the initial situation takes place in similar manner, making use of the auxiliary circuit CA whose auxiliary switch T2 is triggered, in the same manner as described above for the auxiliary switch T1. In addition, the switch S1 is then likewise caused to cease conducting. The voltage at the outlet S then passes from +V to 0 volts in an oscillation that ends with the auxiliary switch T2 ceasing to conduct, with the diode D2 then becoming conductive under the effect of the current I.sub.L.
The converter arm could be used in ways other than that described, and in particular as shown in FIG. 2 where the load is connected between two converter arms identical to the arm of FIG. 1, with the elements of the second arm being given the same references as those of the first, but associated with the prime symbol In addition, and in a variant, the inductors LA and LA' can be connected between the auxiliary switches and the point M instead of being connected between the auxiliary switches and the point S; that changes nothing from the point of view of operation described above. Other modes of operation can be applied to a two-arm converter as shown in FIG. 2, in particular it can operate as an inverter. The operation of each of the two arms is based on that described above. When operating as a chopper, the operation of the left arm implies in alternation the diode D2 and the switch T1, whereas the operation of the right arm implies, synchronously, the diode D1' and the switch T2'. When operating as an inverter, after a positive half-cycle, or a plurality of portions of a positive half-cycle, as described above, operation of a negative half-cycle will involve on one side the diode D1 and then the switch T2, and on the other side the diode D2', and then the switch T1.
In these various cases, the voltage of the output S of one arm of the converter switches over without loss, providing the voltage on said outlet S does indeed go from 0 to +V volts, i.e. providing the voltage across the terminals of the auxiliary circuit CA is reversed, i.e. providing the voltage at the point M is indeed equal to +V/2. For this to be the case, the capacitances of the capacitors CV1 and CV2 of the voltage divider DC must be large enough, the symmetry of the components involved in the conduction stages of the auxiliary circuit must be almost perfect, as must the resistive losses in said auxiliary circuit. Experience shows that these conditions are difficult to satisfy, and that constitutes a problem.
Any imperfection in the above gives rise to the voltage at the point S at the end of the half-cycle under consideration of the oscillation not being +V/2, but being a smaller voltage, such that the potential difference between the terminals of the main switch S1 is not zero with the switch being made conductive even though a residual potential remains across its terminals, thereby giving rise to energy being dissipated in the main switch, which is a drawback insofar as the resulting heat must be dumped and the efficiency of the converter is correspondingly reduced.
Nevertheless, since such imperfections are inevitable, a conventional solution is to accept them and to dimension the power converter accordingly. Nevertheless, that gives rise to the additional problem of determining when the main switch should be made conductive; given that the voltage across its terminals does not reduce to zero, there is no way in which the voltage across its terminals can be detected as passing through zero in order to trigger conduction thereof, which technique would present automatically the advantage of switching without loss. The conventional solution to this additional problem consists in making the main switch conductive when the voltage across its terminals drops below a predetermined voltage threshold. The advantage then lies in being certain that losses will not exceed a defined maximum and so the converter will not be overloaded.